To me, it is a small job in addition to a primary job, and usually one that has a lot of leeway. So working 10 hours at a shoe store, when they set the hours, would be less of a side hustle than tutoring, even if the tutoring was for an online company. (So I don't think a side hustle has to be a self-employment thing.)
But if someone called those 10 hours at the shoe store a side hustle, I wouldn't think it was inaccurate. That's just not how I'd frame it.
As for how you get one, really all you need to do is figure out a way to make money. I believe at one time MMM was making Crossfit platforms for jumping, and sold a few. He had a skill and found a market for it. If you have a crafting hobby, find a way to monetize it (craft fairs, etsy, etc.). (With crafts, you really need to run the numbers to make sure you are actually making money as many crafters don't and what they do for income ends up actually costing them money.). Maybe you have a garden and can sell some of your produce. Perhaps you are good with a certain subject and can do tutoring either by advertising in CL and around local schools or with an online company. Maybe you can find a few people who need an occasional (or regular) baby sitter. Or maybe your work skill is something you can sell on the side as a consultant in some way. Or perhaps several of these.